This invention relates to an improved mordant solution which is particularly suitable for dyeing cellulose fibers, such as cotton and linen, without the use of dangerous and polluting heavy metal salt or iron mordants. The method of this invention further includes a process for dyeing such fibers using natural dyes and nonpolluting mordants.
The use of synthetic dyes began with Perkin's accidental synthesis of mauvein (C.sub.27 H.sub.24 N.sub.4) or "aniline purple" in 1856. Perkin was attempting to synthesize quinine, but recognized the commercial potential of mauvein as a synthetic dye. The synthetic dye industry was the major synthetic organic chemical industry in the early 1900's. Synthetic dyes have now virtually replaced natural or vegetable dyes the textile industry, relegating natural dyes to use by hobbyists and cottage industries. The annual global sales of synthetic dyes are now estimated to be about $20,000,000,000.
Production and use of synthetic dyes however produces serious toxic waste problems. The dye industry has recently attempted to reduce toxic effluents by recharging dyebaths, using different dye techniques that are more efficient, recycling and recovering waste, and using computer technology to control the introduction of dyes and chemicals in the bath. Exhaustion rates have been improved in certain facilities to above about 65%; however, the average exhaustion rate remains about 40% and the synthetic dye industry continues to be a major source of pollution, discharging heavy metal salts and other toxic waste into the environment.
Because of the problems associated with synthetic dyes, there has been a renewed interest in natural or vegetable dyes. Natural dyes, such as indigo, tannin and madder have been used since the beginning of recorded history; however, it is almost impossible using known techniques to obtain the same dye shade twice in succession with natural dyes, even using the same method. As noted by a major synthetic dye manufacturer, the color-giving molecules in the plants used for dyeing have not been "specifically designed by nature" for transfer to a substrate. Using prior natural dyeing processes, it is often necessary to choose conditions which severely damage the fiber to obtain a suitably dyed fabric. Further, it has not been possible to obtain dyed fabrics which are light and washfast and it has not been possible to obtain a full palette of colors, using natural dyes. Thus, the textile industry has generally rejected natural dyes for commercial applications.
The most commonly used mordants for natural dyes are also potential sources of toxic waste. Mordants are chemicals that are necessary to chemically fix most natural dyestuffs. The mordant combines with both the dye molecule and the fiber molecule, producing a permanently fixed insoluble "color lake." The insoluble mordant-dye complex that is chemically combined with the fiber in a mordant dyed fiber is referred to as a color lake. Color lakes are produced with adjective dyes. Color lakes produced by reacting a dye with a metallic salt, such as madder (alizarin) with alum, were also used in inks and paints. The most commonly used mordants for natural dyes are alum or potassium aluminum sulfate, chrome or potassium dichromate or potassium bichromate, blue vitriol or copper sulfate, ferrous sulfate, stannous chloride, sodium dithionite or sodium hydrosulfite, ammonia hydroxide, cream of tartar or potassium bitartrate, "Glauber's salt" or sodium sulfate, lime, lye or sodium hydroxide, oxalic acid, tannic acid, uria, vinegar or acetic acid and washing soda or sodium carbonate. As will be understood, several of these mordants produce toxic waste, but the prior art has failed to produce permanently dyed cellulose fibers or fabric which are color and washfast and which produce a wide range or palette of colors, even using heavy metal salt mordants.
Thus, there is an urgent need for a natural dye process which can meet the needs of the commercial textile industry, including a full palette of consistently reproducible natural colors, which are wash and lightfast. Further, there is an urgent need for a mordant and dye process which does not produce toxic wastes. The mordant and dye process of this invention meets both of these urgent needs.